The present invention relates to apparatus for obtaining samples of earth formations and, more particularly, to new and improved core-sampling apparatus for obtaining a sample of earth formation surrounding a borehole.
There is a need in the oil and gas industry for a means for obtaining intact samples representative of the earth formations at various elevations of a borehole. Such samples, commonly referred to as "cores", may have structures ranging from relatively hard to those which are soft and fragile. Retrieved core samples may be analyzed to determine the nature of the fluids, whether gas, oil or water, with which they are saturated. Further, certain physical characteristics of the formation, such as permeability and porosity, may be determined.
A technique for obtaining core samples of the formation involves the use of a wireline-type instrument for selectively taking samples from the wall of the borehole of any desired level. This tool, referred to as a "Sidewall Sampler", generally includes an elongated body member containing along its length a plurality of sample receiving projectiles. The projectiles each consist of a hollow cylinder having an open end and capable of receiving a sample of the material of the side wall formation upon shooting the cylinder into the formation.
The projectiles are shot into the formation by igniting a propellant charge placed in the instrument behind the projectile. The propellant charges are detonated electrically at the desired level within the borehole by remote control. The projectiles are attached to the housing of the instrument by means of small cables, so that after they have been fired they may be retrieved, with the core sample therein, upon raising the instrument to the surface.
It has been shown that the design of the projectile is influenced by the hardness of the formation from which the core is desired. One prior art design attempts to facilitate recovery of the core projectile by providing a core barrel with a separable forward portion in the form of an annular cutting ring which generally has a slightly greater diameter than the core barrel. When a core barrel of this design enters a formation the enlarged annular ring creates a core hole slightly larger than the core barrel. Upon withdrawal of the core barrel from the formation, the core barrel separates from the annular ring and is easily extracted since the core hole is slightly larger than the core barrel. Such prior art core barrels have been shown to be less than ideal in that the annular ring will at times separate from the core barrel prior to contacting the formation or the ring will stick on the core barrel within the formation making core barrel retrieval a difficult task.
Another prior art core barrel has attempted to solve the retrieval problem by providing a relatively heavy core barrel with a large frontal cutting area. Upon penetration of the formation this core barrel fractures the surrounding formation thereby reducing the resistance to the extraction of the barrel. However, quite often the fracturing of the surrounding formation also causes fracturing of the core sample. Such fracturing can result in large portions of the core sample falling from the barrel during recovery. Another problem with a relatively massive core barrel is that the size of the charge required to cause the needed formation penetration is unnecessarily large.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing method and apparatus for obtaining subsurface formation samples by the use of a replaceable core sample barrel utilizing a relatively small cutting head and a limited penetration distance.